They are awesome. One of the best things to happen in the last decade or so for more aggressive trail riding. In a way it transforms the capability of a lot of bikes for getting rowdy on the descents. I fully endorse the KS, Fox, Specialized and Rockshox ones.

I suspect a lot of people think lowering the seat is so you can stick your arse out the back easy on super steep rolldowns, but in reality it lets you move around the bike much better on all descents, easier hopping, jumping and manualling etc. Only downside is the weight is pretty killer.